AP Biology 2006-2007

Ch. 15 - Errors of MeiosisChromosomal Abnormalities

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Chromosomal abnormalities Incorrect number of chromosomes

nondisjunction chromosomes don’t separate properly

during meiosis breakage of chromosomes

deletion duplication inversion translocation

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Nondisjunction Problems with meiotic spindle cause errors in

daughter cells homologous chromosomes do not separate

properly during Meiosis 1 sister chromatids fail to separate during Meiosis 2 too many or too few chromosomes

2n n

n

n-1

n+1

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Alteration of chromosome number

all with incorrect number 1/2 with incorrect number

error in Meiosis 1

error in Meiosis 2

AP Biologytrisomy

2n+1

Nondisjunction Baby has wrong chromosome number

trisomy cells have 3 copies of a chromosome

monosomy cells have only 1 copy of a chromosome

n+1 n

monosomy2n-1

n-1 n

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Human chromosome disorders High frequency in humans

most embryos are spontaneously aborted alterations are too disastrous developmental problems result from biochemical

imbalance imbalance in regulatory molecules?

hormones? transcription factors?

Certain conditions are tolerated upset the balance less = survivable but characteristic set of symptoms = syndrome

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Down syndrome Trisomy 21

3 copies of chromosome 21 1 in 700 children born in U.S.

Chromosome 21 is the smallest human chromosome but still severe effects

Frequency of Down syndrome correlates with the age of the mother

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Down syndrome & age of motherMother’s age

Incidence of Down Syndrome

Under 30 <1 in 100030 1 in 90035 1 in 40036 1 in 30037 1 in 23038 1 in 18039 1 in 13540 1 in 10542 1 in 6044 1 in 3546 1 in 2048 1 in 1649 1 in 12

Rate of miscarriage due to amniocentesis: 1970s data

0.5%, or 1 in 200 pregnancies 2006 data

<0.1%, or 1 in 1600 pregnancies

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Genetic testing Amniocentesis in 2nd trimester

sample of embryo cells stain & photograph chromosomes

Analysis of karyotype

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Sex chromosomes abnormalities Human development more tolerant of

wrong numbers in sex chromosome But produces a variety of distinct

syndromes in humans XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female

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XXY male one in every 2000 live births have male sex organs, but

are sterile feminine characteristics

some breast development lack of facial hair

tall normal intelligence

Klinefelter’s syndrome

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Klinefelter’s syndrome

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Jacob’s syndrome male XYY Males

1 in 1000 live male births

extra Y chromosome slightly taller than

average more active normal intelligence, slight learning disabilities delayed emotional maturity normal sexual development

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Trisomy X XXX

1 in every 2000 live births produces healthy females

Why? Barr bodies

all but one X chromosome is inactivated

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Turner syndrome Monosomy X or X0

1 in every 5000 births varied degree of effects webbed neck short stature sterile

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Changes in chromosome structure deletion

loss of a chromosomal segment duplication

repeat a segment inversion

reverses a segment translocation

move segment from one chromosome to another

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AP Biology 2006-2007